Money talks -- and very loudly when a drug company is funding a clinical trial involving one of its products, according to a study released Monday.

UCSF researchers looked at nearly 200 head-to-head studies of widely prescribed cholesterol-lowering medications, or statins, and found that results were 20 times more likely to favor the drug made by the company that sponsored the trial.

"We have to be really, really skeptical of these drug-company-sponsored studies," said Lisa Bero, the study's author and professor of clinical pharmacy and health policy studies at the university.

The research, reported in the online editions of PLoS Medicine, a San Francisco medical journal, focused on studies of six statins -- including Pfizer Inc.'s Lipitor, Merck & Co.'s Zocor and the generic drug Mevacor -- that had already been approved by the Food and Drug Administration. The trials typically involved comparing the effectiveness of a drug to one or two other statins.

"If I'm a clinician or funder of health care, I really want to know within a class of drug which one works better," Bero said. "What our study shows is that depends on who funds the study."

UCSF researchers also found that a study's conclusions -- not the actual research results but the trial investigators' impressions -- are more than 35 times more likely to favor the test drug when that trial is sponsored by the drug's maker.

Drug manufacturers, through the industry's trade group, said the federal government cracks down on biased research.

"Our industry is dependent upon well-designed clinical trials that will pass muster with the FDA," Johnson said.

Mark Gibson is deputy director of the Center for Evidence-Based Policy at Oregon Health & Science University, which reviews existing clinical evidence for drug effectiveness and safety. He called the UCSF study an "important piece of work."

"If Americans really want to be able to have sound evidence on which to base their choice of treatments, they need to think about ways to fund independent research," he said.

About half of the 192 statin trials examined in the study between 1999 and 2005 were funded by drug companies. Bero said drug companies fund up to 90 percent of drug-to-drug clinical trials for certain classes of medication.

About a third of the statin trials did not disclose any funding source. Trials with no disclosed funding source were less likely to favor the so-called test drug than those with industry funding, researchers found.

The researchers found other factors that could affect trial results. For example, pharmaceutical companies could choose not to publish results of studies that fail to favor their drugs, or they could be designed in ways to skew results.

The study found the most important weakness of trials was lack of true clinical outcome measures. In the case of statins, some trials focused on less-direct results such as lipid levels but failed to connect the results with key outcomes such as heart attacks or mortality.

"None of us really care what our cholesterol level is. We care about having a heart attack," Gibson said. "For the drug to be worthwhile taking, it has to be directly related to prevent a heart attack."

The UCSF study was funded by a grant from the California Tobacco Related Disease Research Program.

The study, "Factors Association with Findings of Published Trials of Drug-Drugs Comparison," can be found online at medicine.plosjournals.org.